1. Field of the Invention
The present invention relates to liquid crystal display devices and methods for driving the same. More specifically, the present invention relates to AC driving in active-matrix liquid crystal display devices.
2. Description of the Related Art
Ordinary liquid crystal display devices are driven by AC driving, in order to repress deterioration of the liquid crystal and to sustain the display quality. However, in active liquid crystal display devices, since the characteristics of switching elements, such as the TFTs (thin film transistors) provided for each pixel, are not sufficient, the transmittance of the liquid crystal layer does not become perfectly symmetric for positive and negative data voltages, even when the positive and negative portions of the video signals outputted from the video signal line driving circuit (also referred to as “column electrode driving circuit” or “data line driving circuit”), applying voltages to the video signal lines (column electrodes) of the liquid crystal panel, that is, the positive and negative portions of the applied voltage are symmetric with respect to the potential of the common electrode. Therefore, in driving schemes in which the polarity of the voltage applied to the liquid crystal is inverted at each frame (frame inversion driving scheme), flicker occurs in the display of the liquid crystal panel. Moreover, due to parasitic capacitances Csd (intra-pixel) and Csd (inter-pixel) that occur between video signal lines Lss, Lsn and pixel electrodes Ep, as shown in FIG. 9, each of the pixel values corresponding to the voltage between the pixel electrodes Ep and the common electrode Ec is affected by the potentials of the video signal lines Lss and Lsn, and a stripe-shaped pattern extending in vertical direction (also referred to as “vertical shadow”) may appear on the screen.
In liquid crystal modules used for portable information appliances in which there is a particularly high need to reduce power consumption, such as portable phones, a frame inversion driving scheme has been employed as the AC driving scheme in order to meet this need. However, in recent years, also in portable phones, high-quality display capability has come to be demanded for portable phones due to improved processing performance and more sophisticated utilization, and accordingly, the problems of flicker and vertical shadows need to be addressed.
In order to solve these problems, a driving scheme inverting the polarity at each frame while inverting the polarity of the applied voltage at each horizontal scanning line (also called “line inversion driving scheme”) is employed as an AC driving scheme. However, when the line inversion driving scheme is employed instead of the frame inversion driving scheme, then the frequency with which polarities of the video signals to be applied to the liquid crystal panel are inverted (i.e. the inversion frequency) becomes high, and also the switching frequency of the potential of the common electrode becomes high, due to the reduction of the necessary withstand voltage of the driving IC (integrated circuit). As a result, the power consumption becomes large. Moreover, it is not possible to sufficiently suppress flicker merely by employing the line inversion driving scheme.